a) x^2-4x=0 b)x(x+3)-2x-6=0 c)x^3+27+(x+3)(x-9)=0

$a) x^2 - 4x = 0$

       $x(x - 4)  = 0$

\(\left[ \begin{array}{l}x=0\\x-4=0\end{array} \right.\)

\(\left[ \begin{array}{l}x=0\\x=4\end{array} \right.\)

$Vậy: x = 0  hoặc  x = 4$.

$b) x(x + 3) - 2x - 6 = 0$

    $x(x + 3) - 2(x + 3)= 0$

        $(x + 3)(x - 2)    = 0$

\(\left[ \begin{array}{l}x+3=0\\x-2=0\end{array} \right.\)

\(\left[ \begin{array}{l}x=-3\\x=2\end{array} \right.\)

$Vậy: x =-3  hoặc  x = 2$

$c)    x^3 + 27 + (x + 3)(x - 9) = 0$

      $x^3 + 3^3 + (x + 3)(x - 9) = 0$

$(x + 3)(x^2 + 3x + 9) + (x + 3)(x - 9) = 0$

   $(x + 3)(x^2 + 3x + 9 + x - 9) = 0$

                    $ (x + 3)(x^2 + 4x) = 0$

                        $x(x + 3)(x + 4)  = 0$

$=>* x =0$

      $* x + 3 =0 → x = -3$

       $* x + 4 =0 → x = -4$

$Vậy: x = 0; x = -3  hoặc  x = -4$

Học tốt.